Notes:
One of the cracked cap boletes – post your guesses below. I already know which one it is, confirmed by spores.
This is part of an experiment I am doing this season to collect data on how accurate our macroscopic ID of this group actually is. I predict we can accurately diagnose the species in this complex on macromorphology alone >80% of the time, maybe more.

Proposed Names

Recognized by sight: B. zelleri shouldn’t crack at all (should it?). Between B. truncatus and B. chrysenteron, MykoWeb points out that B. truncatus is: “less likely than B. chrysenteron to develop pinkish tones in cracks near the cap margin, and in our experience the pores bruise blue almost instantly, while in B. chrysenteron, this reaction takes several seconds with the blueing never as intense.”

Proposed Names

Recognized by sight: B. zelleri shouldn’t crack at all (should it?). Between B. truncatus and B. chrysenteron, MykoWeb points out that B. truncatus is: “less likely than B. chrysenteron to develop pinkish tones in cracks near the cap margin, and in our experience the pores bruise blue almost instantly, while in B. chrysenteron, this reaction takes several seconds with the blueing never as intense.”

I will try to do that right now.
There are two scenarios I see occurring here:
1) The species are true species that intergrade morphologically to such a degree that we cannot macroscopically identify them (or even microscopically).
2) The species can be told apart by morphology, but the collections in GenBank were misidentified when they were entered. Who knows how likely this is- there is very poor annotation on such sequences.

Have you tried to analyze existing sequences from Genbank belonging to the american collections of chrysenteron and zelleri?

I made an experiment (and also compared with some european chrysenteron).
I’m not experienced with this stuff, but the picture I saw, was that the american collections had two main lineages, but not corresponding with the names zelleri or chrysenteron. Both names occured in both lineages.
One of them wasn’t too far from the european chrysenteron.

The result I got from aligning the sequences, at least indicated that macrocharacters haven’t been useful for delimiting and ID:ing the species. Perhaps no characters are..?

Basically, I think we are confused about which characters matter. I’ll see what I can do this winter to sort these out, but here are my preliminary guesses/opinions:
1) Degree of cracking – B. zelleri shouldn’t crack strongly in most cases.
2) Stature – B. chrysenteron is a softer-fleshed, less robust mushroom.
3) Spores (only really useful between B. chrysenteron and B. truncatus)

Here’s what I don’t think are reliable characters:
1) Cap color – other than the extreme dark purple-red of a fresh, moist B. zelleri, I think they can intergrade.
2) Degree of blue staining – dependent on age, moisture, temperature, blah blah blah

The spore size doesn’t help differentiate B. zelleri and B. chrysenteron, so I guess I have to retract my statement that I “know” what the above mushroom is. But I will say that my vote goes for B. chrysenteron.

PS – MykoWeb supports Jeff’s assertion below (ie. B. zelleri grows with redwood), but I was surprised that it explicitly states CONIFERS. I routinely find it with oak on the UCSC Campus.

Eastern NA Boletus chysenteron is seldom this dark. B.truncatus is reported to not show as much red flesh in the cracks. I do not know B. zelleri. So I would predict that a 50% correct ID based on macro features. I hope I am wrong because I am a field identifier.